XLT 580 won't rev over 4k

Two weeks ago my 1995 Indy XLT 580 ran great, it sat for about a week and now it's super hard to get started and when it does start it won't rev past about 4000 rmps. I took the carbs apart and cleaned them but that didn't help at all. I rode it around for about 10 minutes and all three plugs looked different. Plug on the clutch side was white, center was oil fouled, and mag side looked brand new. Someone suggested I try running premix cause their might be a problem with the oil injection system. Before I drain the regular gas out of the tank I would like to hear some other thoughts on why the engine might not be revving up. I really need help with my problem so I can get out and go ride while the weathers nice here in Alaska. Thanks.

It's been in the garage for last few days and fuel flows freely at least to the pump. I'm still gonna drain the tank and run premix since I took off the oil lines and I don't want to be pumping just air into when I get it fired up.